Glacier is an archiving service. According to their site "Amazon Glacier is optimized for data that is infrequently accessed and for which retrieval times of several hours are suitable". It's meant for data that you don't want to throw away but don't need to access regularly.

To draw a metaphor with paper documents. If the paper on your desk is RAM, the filing cabinet in your office is your HDD, the departmental records room is Amazon S3, the offsite record archival that takes a week to requisition a box of files is Glacier.